100-year Storm (rainfall)

A 100-year storm is the amount of rainfall measured at certain location, during a specified length of time, that has a 1% chance in any given year of being equaled or exceeded. It is normal in Colorado to experience many storms that locally equal or exceed rainfall defined as the 100-year storm. It is likely that between 100 and 300 "100-year storms" occur somewhere in Colorado in a typical year. This graphic compares 100-year storms to storms of other sizes.

100-year Flood (runoff)

A 100-year storm (rainfall) does not always create a 100-year flood (runoff). Many variables exist that factor into how much water runs off and where it goes. In mapping the 100-year floodplain, officials assume that the 100-year storm occurs uniformly over the entire basin. Therefore, if a 100-year rainfall occurs in only an isolated location, it may not result in a 100-year flood.
Similar to the 100-year storm, the definition of the 100-year flood is the specific streamflow that has a 1% chance in any given year of being equaled or exceeded. That streamflow amount is either determined from:

• long-term streamflow records, such as records for the Poudre River; or
• modeling of the runoff from the 100-year storm.

Learn more about Flood Risk.